Playing on her 27th birthday didn't help Capriati, the runner-up for the third year in a row. She also lost to Serena last year and to Venus Williams in 2001.
Williams earned $393,000. Capriati again settled for runner-up money -- $206,000.
Second-seeded Andre Agassi, bidding for his sixth Key Biscayne title and third in a row, plays fifth-seeded Carlos Moya today in the men's final. Agassi advanced by beating ninth-seeded Albert Costa, 6-2, 6-4.
Capriati started fast but acknowledged that the 85-degree heat -- and Williams' relentless groundstrokes -- took a toll in the two-hour match. By the final game, Capriati was hitting so wildly that Williams had to duck out of the way of one shot.
"It was pretty hot out there," Capriati said. "It's tough chasing down her [shots]."
Said Williams, who improved to 17-0 this season: "I saw she was getting tired. And I was just starting."
Both players consider Key Biscayne their home tournament. But while Williams wore Florida orange, the loud stadium crowd was in Capriati's corner -- and disruptive during at least a couple of points.
"If anything, it motivates me to want to win," Williams said. "I can understand -- it's her third time in the final. Honestly, if she was playing anyone else, I would have liked to see her win."
The Agassi-Costa match looked as if it would be a marathon at first, with an opening game that lasted 17 minutes and 26 points. Agassi won it to break serve, then took a 3-0 lead and was in control the rest of the way.
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Serena in '03
*--* By winning the Nasdaq-100 Open Saturday, Serena Williams maintained her perfect record on the WTA Tour in 2003, at 17-0: AUSTRALIAN OPEN 7-0 Final: d. Venus Williams, 7-6, 3-6, 6-4 OPEN GAZ DE FRANCE 4-0 Final: d. Amelie Mauresmo, 6-3, 6-2 NASDAQ-100 OPEN 6-0 Final: d. Jennifer Capriati, 4-6, 6-4, 6-1